This invention relates generally to pneumatic controllers forming a part of a process control system, and more particularly to an improved controller in which transfer from manual to automatic operation is effected in a "balanceless" and "bumpless" manner.
A pneumatic controller is a compenent in a process control loop which is subject to disturbances, the controller acting in conjunction with other devices to maintain a process variable at a desired value. To accomplish this purpose, the controller receives, in terms of pneumatic signals, both the set value and the process variable, the controller functioning to operate a final element which directly or indirectly governs the process variable.
Pneumatic controllers ordinarily operate automatically to maintain a process condition at a desired level. But in some situations, it becomes necessary to transfer the controller from automatic to non-automatic or manual operation. Thus it may be desirable to control the process condition by means of a manually-adjustable signal.
Most controllers of recent process control systems are capable of transferring manual operation to automatic operation or vise versa in balanceless and bumpless manner. In a pneumatic controller, however, manual operation may not always be tranferred to automatic operation unconditionally but bumpless transfer can be achieved only when preportional band input deviation and manual input just before the transfer satisfy certain conditions. In other words, whether bumps are produced or not during the transfer depends upon the above-mentioned three parameters and operators could have hardly judged the time of transfer intuitively.
In conventional pneumatic controllers, a transfer from automatic mode to manual mode could always be accomplished in bumpless manner, however, in the case of the transfer from manual operation to automatic operation, it could be accomplished only when deviation is small and proportional band is large in percentage. For example, in the case of a proportional band of 50% and a manual output 40%, bumps have been produced with a deviation larger than 30% in such a conventional controller. Bumps during the manual-automatic operation transfer are caused by saturation of the output of the comparator which serves to compare the manual output with the automatic output and enables the automatic output to follow the manual output.
Generally, in force-balance type pneumatic controllers of the prior art such as shown in Japanese patent application No. 76,709 of 1972 (Japanese open patent application No. 33,095 of 1974) comprising a proportional band altering unit, a proportional plus integral operation unit and a comparator, in order to perform manual-automatic transfer in a balanceless and bumpless mode, the force to be transmitted from the proportional band altering unit to the proportional plus integral operation unit is limited within a certain range so as to prevent the force of a force beam in the proportional plus integral operation unit from becoming unbalanced due to saturation of a comparison output from the comparator in the manual operaton mode.
Pneumatic controllers of such a type require bellows and other additional mechanisms as the balanceless and bumpless manual-automatic transfer mechanisms, thus they are complicated in construction and expensive.